1. Field of the Invention
The present invention relates to an optical disk apparatus, and in particular to suppression of skipping due to optical disk defects.
2. Description of the Related Art
Optical disk apparatuses for driving optical disks including CD, DVD, and the like, perform data recording and reproduction while maintaining focused and on-track conditions using focus and tracking servos, respectively. Specifically, a laser beam is irradiated toward the surface of an optical disk, and returned light from the surface is received. Then, the obtained reproduction RF signal is utilized to create focus and tracking error signals. These error signals are in turn utilized in creation of servo signals for driving the actuator to further drive the objective lens of a pick-up to move in the focusing and track width directions.
For CDs, an area where a laser beam remains within a track, namely, a dynamic range of a tracking error signal which is usable as a tracking servo system, is smaller than a half of a track pitch. Therefore, assuming a track pitch of approximately 1.6 μm, a dynamic range of smaller than 0.8 μm results. Because of the small dynamic range of a tracking error signal, chips or dust, if any, on a track may be enough to make it impossible for the servo system to precisely follow the track, causing the servo system to be displaced from the track.
In view of the above, technology has been proposed for separating a tracking servo system upon detection of any track defect on an optical disk and sending a signal, instead of a servo signal, which is sampled and held by a sample and hold circuit to a driving system before detection of the defect.
According to Japanese Patent Publication No. Hei 2-34091, for example, a detection circuit for detecting chips, dust, and so forth, on a disk is provided, and a DC level of a tracking error signal is sampled upon detection of chips, dust, and so forth. The DC level is held while the detection is continued until the chips, dust, and so forth are no longer detected, at which point normal control using a tracking servo is resumed.
However, with this arrangement in which a tracking servo system is separated upon detection of a defect and a DC level signal which is sampled immediately before the detection and held by a sample and hold circuit is supplied as a servo signal to a driving system, low resolution of hardware becomes problematic. Specifically, in a low speed operation at a standard or double speed, for example, in which control such as using the least significant one or two bits of a DA converter is applied due to a small driving voltage, low resolution of hardware (a low sampling bit rate) would make it difficult to sample a precise DC level. That is, in such a case, sampling a DC level may serve to impart a kick signal in a predetermined direction to the tracking servo system. In other words, sampling a DC level may serve as disturbance.
In particular, for optical disk apparatuses capable of high speed recording and reproduction, which is generally set with a large actuator coil voltage in order to accommodate a high speed operation, coverage of a possible driving voltage range by a limited number of bits of a DA converter, such as 256 bits, for example, may result in assignment of a relatively large range of voltage to one step. As a result, a small driving voltage used in a low speed operation cannot be properly dealt with and the position of an objective lens thus cannot be precisely held. This causes the servo system to be displaced from the track when it passes a defect.
FIG. 7 shows an example servo signal for an optical disk apparatus. FIG. 7A shows a detection signal supplied from a defect detection circuit, which remains low for absence of defect and high for presence of defect. Such a signal can be detected, as described in Japanese Patent Publication No. Hei 2-34091, based on the level of an amplitude of a reproduction RF signal.
FIG. 7B shows a signal waveform obtained in an operation in which a tracking servo signal is created to be output based on a tracking error signal for a normal portion and a servo signal which is sampled immediately before detection of a defect is output instead of a servo signal for a defect portion. A digital hold signal is converted into an analogue signal in a DA converter and then supplied to a driver. With low resolution of the DA converter, a signal of a greater than predetermined value is output even with respect to the least significant bit. That is, when a DC level for the normal portion is small, a DC level which is different from that of a normal portion is output, largely driving the objective lens in a predetermined position and thereby causing the objective lens to be displaced from the track.
Low resolution of hardware can be relatively enhanced by reducing an output dynamic range. However, simple reduction of an output dynamic range in turn induces deterioration of servo performance.